External reflective type liquid crystal display

ABSTRACT

In a liquid crystal display, first and second substrates are provided opposing each other with a liquid crystal layer held between them, the outer surface of the first substrate serves as a light scattering surface having a light scattering characteristic, and a reflector including an irregular reflection surface is provided on the light scattering surface so that the reflection surface is on the light scattering surface side.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an external type liquid crystaldisplay including a liquid crystal cell having a pair of opposingtransparent substrates with a liquid crystal layer held therebetween,and a reflector provided on an outer surface of the liquid crystal cell.The invention more particularly relates to a liquid crystal displayhaving improved visibility by providing the liquid crystal cell with alight scattering characteristic.

[0003] 2. Description of the Related Art

[0004] Liquid crystal displays are generally divided intosemi-transmissive or transmissive type displays having a backlight andreflective type displays. The reflective type liquid crystal displayutilizes only external light such as sunlight and illumination fordisplay and does not use a backlight. This type of display is widelyused for mobile information terminals that should be thin andlightweight, and operate with low power consumption. The reflective typeliquid crystal display includes a reflector that reflects incoming lightfrom the display surface side for display. Some reflectors have a mirrorsurface and others have an irregular surface. The reflector having, anirregular surface is more suitable for obtaining a wider viewing anglethan the reflector with a mirror surface.

[0005]FIG. 4 is a schematic sectional view of a conventional reflectivetype liquid crystal display including a reflector with an irregularsurface. The conventional liquid crystal display 200 includes a liquidcrystal cell 220, and a front light 210 provided in front of the liquidcrystal cell 220 (in the upper part of FIG. 4).

[0006] The conventional front light 210 includes a light guide plate212, and a light source 213 of a cold cathode tube provided at the sideend face 212 a of the light guide plate 212. The light guide plate 212has an emission surface 212 b as a lower surface (the surface on theside of the liquid crystal cell 220), from which light is emitted. Thesurface on the opposite, side to the emission surface 212 b (the uppersurface of the light guide plate 212) is a prism surface 212 c that canchange the optical path of light propagating through the light guideplate 212 toward the emission surface 212 b.

[0007] The liquid crystal cell 220 has first and second opposingsubstrates 221 and 222 joined to each other by a seal member 224 andhaving a liquid crystal layer 223 therebetween. There are displaycircuits 226 and 227 on the first and second substrates 221 and 222 onthe side of the liquid crystal layer 223 (at the inner side). Areflector 230 is provided on the side of the first substrate 221opposite to the liquid crystal layer 223 (at the outer side) through anadhesive layer 231. The reflector 230 includes a reflector substrate 228having one irregular surface, and a reflection layer 229 formed on theirregular surface of the reflector substrate 228. The reflection layer229 is located on the side of the first substrate 221.

[0008] In the liquid crystal display 200 having the above structure,when the front light 210 is turned on, light emitted from the lightsource 213 is propagated in the light guide plate 212 and emitted fromthe emission surface 212 b. The light emitted from the emission surface212 b is let into the liquid crystal cell 220 as the illumination light,sequentially passed through the second substrate 222, the displaycircuit 226, the liquid crystal layer 223, the display circuit 227, thefirst substrate 221, and the adhesive layer 231 and reflected by thereflection layer 229. The reflected light is returned to the outer sideof the liquid crystal cell 220 (the side of the front light 210), andreaches the viewer through the emission surface 212 b and the prismsurface 212 c of the front light 210. In this way, the displayed contentat the liquid crystal cell 220 is viewed by the viewer.

[0009] When sunlight is used for display rather than turning on thefront light 210, sunlight comes into the liquid crystal cell 220 throughthe prism surface 212 c and the emission surface 212 b of the frontlight 210, and light reflected by the reflection layer 229 is viewed bythe viewer similarly to the above described case.

[0010] However, the on going development and designing of the reflectivetype liquid crystal displays including the reflector with the irregularreflection surface aims at forming fine irregularities (raise andrecessed portions) on the reflection surface to adjust the angle of thereflected light and controlling the irregular geometry to control eventhe directivity of the reflected light. The fine irregularities on thereflection surface surely improve the controllability of the reflectedlight, but there is a problem that a spectrum is more likely to occurunder intense sunlight, which could cause a rainbow-like pattern(hereinafter simply as “rainbow”) to be observed on the display screen,in other words, the visibility is lowered.

SUMMARY OF THE INVENTION

[0011] The present invention is directed to a solution to the aboveproblem, and a liquid crystal display including a reflector having anirregular surface that can prevent a rainbow from appearing on thescreen is provided.

[0012] In order to solve the above problem, the liquid crystal displayaccording to the invention includes a pair of opposing transparentsubstrates having a liquid crystal layer held therebetween. An outersurface of one of the transparent substrates serves as a lightscattering surface having a light scattering characteristic. A reflectorhaving an irregular reflection surface is provided on the lightscattering surface so that the reflection surface is on the side of thelight scattering surface.

[0013] The liquid crystal display according to the invention includesthe light scattering surface between the reflection surface and theliquid crystal layer, and therefore light reflected by the reflectionsurface of the reflector is scattered as it passes through the lightscattering surface, despite a spectrum caused at the irregularities atthe reflection surface. Therefore, the rainbow is prevented from beinggenerated at the display screen.

[0014] According to the invention, the light scattering surfacepreferably has irregularities.

[0015] In this manner, the outer surface of one of the transparentsubstrates on the side (light scattering surface) having the reflectoris provided with irregularities, so that the surface may have apreferable light scattering characteristic.

[0016] According to the invention, the haze of the light, scatteringsurface is preferably in the range from 15% to 30%.

[0017] As will be described, the haze is an index of the degree of lightscattering. When the outer surface (light scattering surface) of theabove-mentioned one transparent substrate has too small a lightscattering characteristic, there is not a sufficient rainbow eliminationeffect at the display screen. Meanwhile, when the light scatteringcharacteristic is too large, the display characteristic at the screen issignificantly lowered. When the haze at the outer surface (lightscattering surface) of the above mentioned one transparent substrate isin the range from 15% to 30%, the display characteristic of the liquidcrystal display can be restrained from being lowered, while the rainbowat the display screen can be prevented.

[0018] According to the invention, the reflector is provided on thelight scattering surface through the adhesive layer and the differencebetween the refractive index of one transparent substrate having thelight scattering surface and the refractive index of the adhesive layeris preferably at least 0.01.

[0019] In this way, the refractive indexes of the light scatteringsurface and the adhesive layer can be different, so that light reflectedby the reflection surface of the reflector can effectively be scatteredat the light scattering surface, and the rainbow can be prevented frombeing generated.

[0020] The thickness of one transparent substrate having the lightscattering surface is preferably smaller than the thickness of the othertransparent substrate.

[0021] The illumination light passed through the liquid crystal layernear the reflector is let into the above mentioned one transparentsubstrate, passed the substrate and then reflected by the reflector. Thereflected light is passed once again through the above describedtransparent substrate and then passed through the liquid crystal layer.

[0022] When the above mentioned one transparent substrate provided nearthe reflector is made thinner, the display can become even thinner andlighter, and the transmission loss as the light passes through thetransparent substrate can be reduced.

[0023] When the above mentioned one transparent substrate is madethinner, the shift between the position where the light passes beforethe reflection and the position where the light reflected by thereflector passes through can be smaller at the interface between theliquid crystal layer and the transparent substrate. Therefore, clearerdisplay can be provided. When in particular a color filter is providedon the transparent substrate that is thin, the shift between theposition of the color filter where the light passes before thereflection, and the position of the color filter where the illuminationlight reflected by the reflector passes can be smaller. Therefore, colorshift and parallax can be reduced and high definition color display canbe carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a perspective view of a liquid crystal display accordingto one embodiment of the invention;

[0025]FIG. 2 is a sectional view taken along line 2-2 in FIG. 1;

[0026]FIG. 3 is a perspective view of a main part of a reflectoraccording to the invention; and

[0027]FIG. 4 is a sectional view of a conventional reflective typeliquid crystal display.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Now, a liquid crystal display according to one embodiment of theinvention will be described in conjunction with FIGS. 1 and 2. FIG. 1 isa perspective view of the liquid crystal display according to theembodiment. FIG. 2 is a sectional view taken along line 2-2 in FIG. 1.

[0029] The liquid crystal display 1 according to the embodimentessentially includes a liquid crystal cell 20, a front light 10 providedon the viewer side of the liquid crystal cell 20, and a reflector 30externally provided on the liquid crystal cell 20 on the side oppositeto the side of front light 10.

[0030] The type of the front light 10 is not particularly limited, and atranslucent, surfacer light emitting member in an arbitrary shape may beused. According to the embodiment, the front light 10 includes atransparent light guide plate 12 made of acrylic resin, for example, anda light source 13 of a cold cathode tube provided at the side end face12 a of the light guide plate 12. The lower surface of the light guideplate 12 (the surface on the side of liquid crystal cell 20) forms asmooth emission surface 12 b from which light is emitted. The surface onthe side of the light guide plate 12 opposite to the emission surface 12b (the upper surface of the light guide plate 12) forms a prism surface12 c having a plurality of wedge shaped grooves in a stripe shape atprescribed pitches. The grooves are provided to change the direction oflight propagating in the light guide plate 12.

[0031] The liquid crystal cell 20 essentially includes first and secondopposing substrates 21 and 22 having a liquid crystal layer 23 heldtherebetween. The substrates are integrally joined by a seal member 24.

[0032] The first and second substrates 21 and 22 are made of atransparent substrate such as a glass substrate, and there are displaycircuits 26 and 27, respectively on their liquid crystal layer 23 sides(at their inner sides).

[0033] The display circuits 26 and 27, though not shown, may include forexample an electrode layer made of a transparent conductive film todrive the liquid crystal layer 23, and an alignment film to control thealignment of the liquid crystal layer 23. When color display is carriedout, the circuits may include a color filter.

[0034] The second substrate 22 provided on the viewer side has its bothsurfaces (the inner and outer surfaces 22 a and 22 b) finished into amirror surface. The second substrate 22 preferably has a thickness aboutin the range from 0.3 mm to 1.1 mm.

[0035] Meanwhile, the first substrate 21 on the side of the reflector 30is thinner than the second substrate 22 and has its inner surface 21 afinished into a mirror surface and its outer surface 21 b formed to haveirregularities. The first substrate 21 preferably has a thickness aboutin the range from 0.1 mm to 0.7 mm.

[0036] The first substrate: 21 is preferably formed by grinding orchemically etching the entire one surface (outer surface 21 b) of atransparent substrate identical to the second substrate 22 so that thepart is thinned and the surface becomes rough. In a particularlypreferable manner, the liquid crystal cell 20 is assembled using twoidentical transparent substrates, and then one of the transparentsubstrates has its outer surface ground before the reflector 30 isattached.

[0037] The outer surface 21 b of the first substrate 21 is rough andprovided with fine irregularities, which makes the surface scatterlight, in other words, the surface serves as a light scattering surface.

[0038] The degree of light scattering at the surface can be controlledby changing the grinding or chemical etching conditions and thuschanging the surface roughness. The degree of scattering of light at thelight scattering surface is preferably in the range from 15% to 30% interms of haze, more preferably from 20% to 30%, even more preferablyfrom 20% to 25%.

[0039] Herein, the haze is the ratio of diffuse transmittance (%) to thetotal light transmittance (%) and used as an index of the degree oflight scattering. The value of haze according to the invention isobtained by a measuring method based on JIS K 7105.

[0040] According to the embodiment, when the haze of the outer surface(light scattering surface) 21 b of the first substrate 21 is less than15%, the rainbow on the display screen cannot be eliminated aseffectively as intended. Conversely, when the haze is more than 30%,there is too much scattering of light, which lowers the reflectionefficiency of the illumination light, and the screen could be darkened,or the contrast on the display screen could be lowered, in other words,the display characteristics can significantly be degraded.

[0041] The reflector 30 has irregularities on the reflection surface,reflects and diffuses incoming light. According to the embodiment, thereflector includes a reflector substrate 28, one surface of which is anirregular surface, and a reflection layer 29 provided on the irregularsurface of the reflector substrate 28.

[0042] The reflector 30 is adhesively joined with the outer surface 21 bof the first substrate 21 under the liquid crystal cell 20 through anadhesive layer 31 so that the reflection layer 29 is directed to theside of the liquid crystal cell 20.

[0043]FIG. 3 is a perspective view of a main part of the reflector 30used preferably according to the embodiment. In the reflector 30, thereflector substrate 28 has a substrate 28 a made of glass and aplate-shaped resin base member 28 b made of a photosensitive resin layeror the like placed on the substrate 28 a. A large number of recesses 35whose inner surfaces form a part of a sphere are continuously formed onthe surface of the resin base member 28 b. The reflection layer 29 madeof a thin metal film is provided on the surface.

[0044] The reflector 30 is, produced for example by forming a resinlayer of photosensitive resin on the substrate 28 a, thenpress-attaching a transfer pattern made of silicone-based resin havingan irregular surface in an inverted shape to the reflection surface ofthe reflector 30, and then curing the resin layer to form the resin basemember 28 b. A metal material having a high reflectance such as aluminumand silver is used to form the reflection layer 29 on the resin basemember 28 b by sputtering or vacuum vapor deposition.

[0045] Preferably, the recesses 35 on the surface of the reflector 30are formed to have a random depth in the range from 0.1 μm to 3 μm andat a pitch in the range from 5 μm to 100 μm between adjacent recesses35. The inclination angle of the inner surface of the recesses 35 ispreferably set in the range from −30° to +30°. It is particularlyimportant that the inclination angle distribution of the inner surfaceof the recesses 35 is in the range from −30° to +30° and that the pitchof adjacent recesses 35 is randomly set in all the directions of theplane. This is because the more regular the pitch of adjacent recesses35 is, the more easily light interference can occur and the moreintensely the reflected light can be colored. When the inclination angledistribution of the inner surface of the recesses 35 is outside the −30°to +30° range, the diffusion angle of the reflected light is too broad,which lowers the reflection intensity and sufficiently bright displaydoes not result. More specifically, the diffusion angle of the reflectedlight is not less than 36° in the air, the reflection intensity peak inthe liquid crystal display is lowered, and the reflection lossincreases.

[0046] When the pitch of adjacent recesses 35 is less, than 5 μm, themanufacture for a transfer pattern for the resin base member 28 b mustbe restricted, and the working period can be much prolonged. A geometrysufficient to provide desired reflection characteristics cannot beformed or optical interference might be caused. In practice, thetransfer pattern to form the surface geometry of the resin base member28 b is produced using a transfer pattern base member having manydiamond indenters press-attached to the member. The tip end size of thediamond indenters is preferably in the range from 20 μm to 200 μm, andtherefore the pitch of adjacent recesses 35 is preferably in the rangefrom 5 μm to 100 μm.

[0047] Note that herein, the “depth of the recess” refers to thedistance from the front surface of the reflector 30 to the bottom ofthee recess 35, the “pitch of adjacent recesses” refers to the distancebetween the centers of the recesses 35 each forming a circle when thesurface of the reflector 30 is viewed two-dimensionally. The“inclination angle” refers to the angle of the tangent to the recess 35in an arbitrary location of the inner surface in a particular verticalsection with respect to the substrate surface.

[0048] The adhesive layer 31 is made of a transparent resin adhesivesuch as fluorine-containing epoxy-based resin. The adhesive layer 31 ismade of a material having a refractive index different from that, of thefirst substrate 21 adjacent to the adhesive layer. If the refractiveindexes of the adhesive layer 31 and the first substrate 21 are equal,light is not scattered at the outer surface 21 b (light scatteringsurface) of the first substrate 21, which is the interface between thefirst substrate 21 and the adhesive layer 31. Therefore, in order toeffectively eliminate the rainbow on the display screen as intended byscattering light at the outer surface 21 b (light scattering surface) ofthe first substrate 21, the difference between the refractive indexes ofthe adhesive layer 31 and the first substrate 21 is preferably at least0.01. When the difference between the indexes is too large, thereflection characteristic largely departs from the designed value, andtherefore the difference is preferably not more than 0.2.

[0049] When for example the first substrate 21 is made of glass, therefractive index is about 1.52, and therefore the material for theadhesive layer 31 may be a resin-material having a refractive index inthe range from about. 1.32 to 1.72 such as acrylic resin (refractiveindex: 1.46), fluororesin (refractive index: 1.34), and epoxy resin(refractive index: 1.61).

[0050] The liquid crystal display 1 having the above-described structurecan carry out reflective display using the ambient light such assunlight and illumination as the illumination light and reflectivedisplay using light from the front light 10 as the illumination light.

[0051] In any of the cases, the illumination light coming into theliquid crystal cell 20 passes through the second substrate 22, thedisplay circuit 26, the liquid crystal layer 23, the display circuit 27,the first substrate 21 and the adhesive layer 31, and then reflected bythe reflection layer 29 of the reflector 30. The reflected light passesthrough the adhesive layer 31 and is let into the liquid crystal cell 20from the outer surface 21 b of the first substrate 21 in the reversedorder from the illumination light, and then emitted from the outerside(the side of the front light 10) of the liquid crystal cell 20. Then,the light passes through the front light 10 and reaches the viewer, sothat the display at the liquid crystal cell 20 is viewed by the viewer.According to the embodiment, when intense sunlight comes in, forexample, and the illumination light is reflected by the reflectionsurface (reflection layer 29) of the reflector 30, the light isscattered as it passes through the outer surface 21 b of the firstsubstrate 21 serving as the light scattering surface despite a spectrumcaused at the irregularities at the reflection surface. Therefore, therainbow is not observed by the viewer.

[0052] The haze of the outer surface 21 b of the first substrate 21 asthe light scattering surface is set in the above preferred range, sothat the rainbow at the display screen can be prevented, while thescreen can be prevented from being darkened or a drop in the contrastcan be prevented that would otherwise be caused by too much scatteringof light and reduction in the reflectance of the illumination light as aresult.

[0053] Since the first substrate 21 is thinner than the second substrate22, a transparent substrate identical to the second substrate 22 may beground or chemically etched, so that the part has a smaller thickness oris provided with a rough surface. In this way, the substrate can beproduced readily and inexpensively. The liquid crystal cell is assembledusing the two identical transparent substrates, and then one of thetransparent substrates may be ground, so that the grinding step needsonly be added to the conventional method of manufacturing the liquidcrystal cell. This does not complicate the work and high manufacturingefficiency is secured.

[0054] The first substrate 21 is reduced in thickness, so that theliquid crystal display 1 can be reduced in thickness and weight. Thetransmission loss of the light passing through the first substrate 21can be reduced as well.

[0055] Note that if necessary a retardation film or a polarizer may beprovided on the second substrate 22 on the side of the front light 10 ofthe liquid crystal cell 20.

[0056] When a color filter is provided at the display circuit 27 on thefirst substrate 21 on the side of the reflector 30, color display can becarried out. The first substrates 21 interposed between the color filterand the reflector 30 is formed to be particularly thin, so that highdefinition color display with reduced color shift or parallax can beprovided.

[0057] In addition, according to the embodiment, the outer surface 21 bof the first substrate 21 has the irregularities in order to have alight scattering characteristic. The outer surface 21 b of the firstsubstrate 21 can be formed into a light scattering surface by othermethods. For example, the light scattering characteristic can beprovided by attaching a film with a scattering effect to the outersurface 21 b of the first substrate 21.

EXAMPLE

[0058] Now, a specific example of the present invention will bedescribed in order to disclose the effect of the invention, but theinvention is not limited to the example.

Example 1

[0059] A liquid crystal cell 20 having the structure shown in FIG. 2 wasproduced. Two glass substrates (refractive index: 1.52) as thick as 0.7mm were prepared as the first and second substrates 21 and 22. Thedisplay circuits 26, 27 were formed on respective one surfaces of the,glass substrates, and then the two, glass substrates were opposed sothat the display circuits 26 and 27 were inside. The seal member 24 wasinserted therebetween and these elements were united integrally. Thespace surrounded by the glass substrates and the seal member 24 wasfilled with liquid crystal and sealed, and then the outer surface of theglass substrate to serve as the reflector side was ground, so that thethickness became 0.2 mm. The grinding condition was set so that the hazeof the ground surface attained a prescribed value.

[0060] Meanwhile, the reflector 30 having the structure shown in FIG. 3was produced. More specifically, a resin layer of acrylic photosensitiveresin was placed on one surface of a substrate 28 a made of glass asthick as 0.7 mm, and then a transfer pattern of silicone-based resin waspress-attached on the surface of the resin layer to form recesses 35followed by irradiation with a ultraviolet beam for curing. In this way,the resin base member 28 b was formed. A reflection layer 29 of aluminumhaving a thickness of 1300 Å was formed on the resin base member 28 b,and the reflector 30 was thus obtained.

[0061] The reflector 30 was adhesively joined on the ground surface(outer surface 21 b of the first substrate 21) of the above liquidcrystal cell 20 through acrylic resin (refractive index: 1.46) so thatthe surface having the recesses 35 was on the side of the liquid crystalcell 20. A liquid crystal display was assembled using this.

[0062] The condition of grinding the outer surface of the glasssubstrate (first substrate 21) of the liquid crystal cell 20 positionedon the side of the reflector 30 was varied, while the other conditionswere unchanged and liquid crystal displays were manufactured with thehaze of the ground surface being varied in the range from 5% to 40% in5% steps.

[0063] The obtained liquid crystal displays were evaluated for thedisplay characteristics. The evaluation items of the displaycharacteristics were the effect of rainbow elimination, the reflectance,the contrast and the total evaluation. The result is given in Table 1.

[0064] For the effect of rainbow elimination, sunlight was let into eachliquid crystal display, and the display screen was observed from all thedirections. In the table, O represents the case when no rainbow wasobserved, Δ when a rainbow was slightly observed, and x when an intenserainbow was observed.

[0065] The reflectance was obtained by measuring the quantity of lightat an incident angle of −30° and an acceptance angle of 0° with respectto the normal to the display screen. In the table, O represents the casewhen the measured value was not less than 90% of the value at a haze of0%, Δ when the value was more than 80% and less than 90% of that value,and x when the value was not more than 80% of that value.

[0066] For the, contrast, the ratio of quantities of light between whenvoltage is applied and when no voltage is applied at an incoming angleof −30° and a receiving angle of 0° was appreciated. In the table, Orepresents the case when the ratio was not less than 80% of the value atthe haze of 0%, and x when the ratio was smaller.

[0067] For the total evaluation, O represents the case when the resultsfor the effect of rainbow elimination, the reflectance and the contrastwere all O, Δ when O was not obtained for all the items, but there wasno x, and x when there was x for at least one item. TABLE 1 rainbowtotal haze elimination reflectance contrast evaluation 5 x ∘ ∘ x 10 x ∘∘ x 15 Δ ∘ ∘ Δ 20 ∘ ∘ ∘ ∘ 25 ∘ ∘ ∘ ∘ 30 ∘ Δ ∘ Δ 35 ∘ x x x 40 ∘ x x x

[0068] As in the foregoing, in the liquid crystal display according tothe invention, a pair of transparent substrates are placed opposing eachother with a liquid crystal layer held therebetween, one of thetransparent substrates has an outer surface serving as a lightscattering surface that scatters light, and a reflector having anirregular reflection surface is provided on the light scatteringsurface. In this way, the reflection surface is on the side of the lightscattering surface. Therefore, when a spectrum is caused by theirregularities on the reflection surface when the illumination light isreflected by the reflector, the reflected light can be scattered as itpasses through the light scattering surface, so that the rainbow at thedisplay screen can be prevented, and a reflective type liquid crystaldisplay with improved visibility can be provided.

What is claimed is:
 1. A liquid crystal display, comprising: a pair ofopposing transparent substrates having a liquid crystal layer heldtherebetween, an outer surface of one of the transparent substratesserving as a light scattering surface having a light scatteringcharacteristic; and a reflector having an irregular reflection surfaceprovided on said light scattering surface so that said reflectionsurface is on the side of said light scattering surface.
 2. The liquidcrystal display according to claim 1, wherein said light scatteringsurface has irregularities thereon.
 3. The liquid crystal displayaccording to claim 1, wherein the haze of said light scattering surfaceis in the range from 15% to 30%.
 4. The liquid crystal display accordingto claim 1, wherein said reflector is, provided on said light scatteringsurface with an adhesive layer therebetween, and the difference betweenthe refractive index of one transparent substrate having said lightscattering surface and the refractive index of said adhesive layer is atleast 0.01.
 5. The liquid crystal display according to claim 1, whereinthe thickness of one transparent substrate having said light scatteringsurface is smaller than the thickness of the other transparentsubstrate.